This is not the movie they showed in the trailers. In the trailers "Black Snake Moan" is the story about some religious fundamentalist who chained up a druggie sex fiend in his house while he tries to drive the sin from her. Seems like a story that stars mostly just two people and their interaction. Could be performed just as well on a stage.
In reality this is the story of two people. One is a middle aged (or just past) black man whose wife has just left him for another man. The other is a late teens white girl whose boyfriend has just joined the army so she's started having sex with everything that moves and doing a lot of drugs.
The first part of the movie covers how their lives fall apart and how poorly they're dealing with it. Then the girl gets dumped in a ditch outside the guy's house after getting beaten by one of the guys who wanted to jump her. The guy brings her in and fixes her up. She keeps wondering about in a delusional haze so he ties a chain to her. When she comes to he doesn't undo the chain. She still "ain't right".
A little time is spent with her making crude comments and him preaching to her. She acts like he's her molesting father and he acts like she's his cheating wife. But with time they both soften and become friends. He gets her to dress better and she gets him to play his guitar in public again, he expresses his grief through the blues.
By the end of the movie he's got a new girl and she's got her boyfriend back and the ability to cope when he's not around.
Not something I'd have bought on DVD, but it's worth watching once.